A possible case of sporadic aurora observed at Rio de Janeiro

Being footprints of major magnetic storms and hence major solar eruptions, mid- to low-latitude aurorae have been one of the pathways to understand solar–terrestrial environments. However, it has been reported that aurorae are also occasionally observed at low latitudes under low or even quiet magnetic conditions. Such phenomena are known as “sporadic aurorae”. We report on a historical event observed by a scientist of the Brazilian Empire in Rio de Janeiro on 15 February 1875. We analyze this event on the basis of its spectroscopic observations, along with its visual structure and coloration, to suggest this event was a possible case of sporadic aurorae. Given the absence of worldwide aurora observations on that day as a consequence of low magnetic activity recorded on the days preceding the observation, in addition to a detailed description, the event observed can most likely be classified as a sporadic aurora. We discuss the geographic and magnetic conditions of that event. Thus, we add a possible case of sporadic aurora in the South American sector.

However, auroral phenomena have been rarely seen at low latitudes during moderate and even quiet magnetic conditions. Such events are known as sporadic aurorae (Silverman, 2003). Botley (1963) introduced this term to the scientific community, citing previous descriptive usages of the same word by Abbe (1895). She introduced nine cases of low latitude aurorae in Europe and in the Middle East observed in the 12th and 19th centuries. Botley (1963) was the first to clearly define sporadic aurorae as comprise such instances as a single ray in a sky otherwise seemingly clear of auroral light, or isolated patches well to the equatorial side of a great display". Botley (1963) also noted references to reports of two low-latitude aurora occurrences without the occurrence of high-latitude aurorae (Eddie, 1894;Fritz, 1881).
It took another four decades for the next paper on sporadic aurora to be published. Silverman (2003) provided a survey of considerable sporadic aurora observations in low-latitude regions of the United States during a time span of over half a century, and highlighted the occurrence of sporadic aurorae in the context of mid-to low-latitude aurorae during moderate to low magnetic activity. That paper was later followed by other papers with reports on sporadic aurora sightings from Iberia and the Canary Islands (Vaquero, Trigo, & Gallego, 2007;Vázquez & Vaquero, 2010), East Asia (Willis, Stephenson, & Fang, 2007), Mexico (Vaquero, Gallego, & Domínguez-Castro, 2013), and the Philippines (Hayakawa, Vaquero, & Ebihara, 2018). Interestingly, Shiokawa et al. (2005) reported three cases of instrumental observations of mid-latitude aurora in Hokkaido (Japan) under fairly moderate magnetic activity as well. Silverman (2003) speculated that sporadic aurorae may be caused by localized and ephemeral magnetospheric energy input into the low-latitude ionosphere, but he does not clearly suggest any physical mechanisms that may explain this phenomenon. In fact, considering the known correlation between intensity of magnetic disturbance and equatorward boundary of auroral ovals (Yokoyama, Kamide, & Miyaoka, 1998), Silverman (2003's comprehensive survey was striking and casted an open question on its physical mechanism. Hayakawa, Vaquero, and Ebihara (2018) suggested that at least part of sporadic aurorae might have been caused by the impact of inclined interplanetary shocks (see also Oliveira & Samsonov, 2018) that strike the magnetosphere in the pre-dusk sector. However, despite all these efforts, a comprehensive understanding of the causes of sporadic aurorae still remains an open question in space weather research.
The main goal of this article is to show an aurora observation report published in a Rio de Janeiro's newspaper on 17 February 1875, hitherto unknown to the scientific community. Based on the event descriptions, the expertise of the observer, and the sporadic aurora characteristics presented in this introduction, as well as the magnetic latitude location of Rio de Janeiro and the low magnetic activity on the days before the observation, we will show that the event was most likely a sporadic aurora. The paper is structured as follows. Section 2 brings brief descriptions of the observational site and the observer. Section 3 introduces the report along with its interpretation based on current aurora knowledge. Finally, the paper is concluded in section 4 along with a final remark.
2 The observational site and the observer 2.1 The Imperial Observatory Brazil was a Portuguese colony during the period 1500 to 1822. Due to military and commercial sanctions imposed by Napoleon to Lisbon in the beginning of the 19th century, the throne of the Portuguese Empire exiled from Lisbon to Rio de Janeiro in 1808 (Fausto, 1994). Later, John VI of Portugal returned back to Lisbon and left his son Peter I as the ruler of the Kingdom of Brazil. Then, on 7 September 1822, Peter I proclaimed Brazil's independence of Portugal, and became the first emperor of Brazil (Fausto, 1994). In 1827, seven years before his death, Peter I founded the Imperial Observatory (Morize, 1987), today known as the National Observatory (Observatório Nacional), still located in Rio de Janeiro. After Peter I's death, his son Peter II became the second and last emperor of Brazil, when it became a Republic on 15 November 1889 (Fausto, 1994). Peter II was a monarch very interested in science who supported many contemporary scientists, and used the auspices of the Imperial Observatory for astronomical observations and scientific discussions (Benevides, 1979).

Emmanuel Liais
The likely sporadic aurora reported here was observed from the facilities of the Imperial Observatory by the Frenchman Emanuel Liais (1828-1900) on 15 February 1875. Liais was the director of the Imperial Observatory in 1875, having been directly appointed by Peter II, after leaving the position as the director-adjunct of the Paris Observatory in France (Morize, 1987). The observer was a professional 19th century scientist. While at the Imperial Observatory, Liais conducted research on astronomy with emphasis on planetary motion and comets, discovering one himself in 1860 (Liais, 1860). He also published a popular book on astronomy (Liais, 1865). Liais had considerable experience and expertise with optical physics and instrumentation. He published on the 1858 total solar eclipse observation from Brazil, being among the first to photograph the solar corona (Aubin, 2016;Liais, 1861), and apparently had a good understanding of atmospheric effects with respect to their altitude occurrences (Liais, 1859;Muniz Barreto, 1997). Liais also published on aurora observations from his home town Cherburg, France, on the Halloween day of 1853 (Liais, 1853). More surprisingly, Liais even suggested methods to measure auroral altitudes, showing that aurorae occur far higher than meteorological phenomena (Liais, 1851), as is well known today (e.g., Roach, Moore, Bruner Jr., Cronin, & Silverman, 1960). According to Muniz Barreto (1997), these findings would have contributed to classify auroral phenomena as magnetic phenomena as opposed to meteorological phenomena if they had been published in a scientific journal with higher audience. 3 The report and its interpretation 3.1 Presentation of the report Emmanuel Liais observed the aurora event on 15 February 1875 from Rio de Janeiro, Brazil. At that time, the Imperial Observatory was hosted by the Morro do Castelo (Castle Hill), an old church whose geographic coordinates are 22.75 • S, 43.10 • W. Liais took notes of his observations and wrote a report to the local Jornal do Commercio (1875) (Commerce Newspaper). This report was found in the data base of the National Digital Library of the National Library of Brazil (http://bndigital.bn.gov.br), hereafter BNDigital). We transcribed the full text of early modern Portuguese with its original spelling and grammar style in Appendix A.1 and translated it into English in Appendix A.2. The observer noted the occurrence of the aurora by 19:45 local mean time (LMT), or ∼ 16:45 GMT (Greenwich Mean Time). He realized the presence of white light in the sky that was aligned with the terrestrial magnetic field. Later, he noted that the light rays, with variable intensities, moved from west to east and took some reddish color at the bottom and greenish color at the top. This display lasted for approximately 40 minutes.
While we certainly need to be careful for possible misinterpretation of atmospheric optics as aurorae (e.g., Stephenson et al., 2019;Usoskin, Kovaltsov, Mishina, Sokoloff, & Vaquero, 2017), there are three more descriptions that strongly suggest that the phenomenon observed by Liais was a sporadic aurora. First, he mentions that some clouds later passed below the auroral rays. Since Liais knew that the aurora is formed above meteorological phenomena occurring in the troposphere (Liais, 1859;Muniz Barreto, 1997) based on his previous experience with aurora observations (Liais, 1853(Liais, , 1859(Liais, , 1865, here it seems that he was very convinced the lights he observed were indeed auroral lights rather than meteorological phenomena associated with clouds. Secondly, its reported direction and coloration seem to rule out this kind of possible contamination. This phenomenon appeared from the direction of the magnetic needle inclination and generated white stripes in a meridian direction. This is consistent and typical with auroral ray structure, extending along the magnetic field line (e.g., Chamberlain, 1961). This phenomenon shows reddish color and faint greenish color well after sunset (18:40 LMT), whereas the nighttime atmospheric optics caused by the Moon is too faint to obtain its color detected by human eyes (Minnaert, 1993).
Even more decisively, Liais saw this phenomenon with his spectroscope and confirmed "the certain evidence of proper lights. Spectroscopic observations frequently give us incontrovertible evidence to distinguish aurora from other atmospheric optics, as auroral spectra show emission lines, whereas spectra of solar reflected lights show absorption lines (see Figure 1;e.g., Capron, 1879e.g., Capron, , 1883Love, 2018;Stephenson et al., 2019). Therefore, we can incontrovertibly reject possible contamination of atmospheric optics or clouds with strange color.
Strangely, Liais interpreted the observed spectra as those of sulfur. This contradicts the modern understanding of auroral spectra as the present day understanding of the aurora shows that there are emissions mainly from oxygen and nitrogen (e.g., Chamberlain, 1961;Gault, Koehler, Link, & Shepherd, 1981). However, it is not Liais originality to associate auroral spectra with sulfur. Back in mid 18th century, van Musschenbroek (1762) suggested aurorae were partially caused by burning sulfur. Liais performed his observations with a spectroscope only 6 years after the earliest spectroscopic observations of the aurora (Ångström, 1869). Even in the 1870's, Capron (1879) acknowledged this early hypothesis of sulfurous vapors issuing from the earth as a cause of the aurora, while Capron himself did not seem to agree with this supposed cause. Moreover, this misinterpretation may be justified by the similarity of spectra of sulfur emissions with spectra of mixed emissions of oxygen and nitrogen, as shown in Figure 1. Since spectrum lines are considered a "fingerprint" of a source or object, this is a very important observation to distinguish aurorae form other optical phenomena. Therefore, most likely Liais was influenced by such early scientific discussions and misinterpreted the compound spectra of excited nitrogen and oxygen emissions as the emission spectrum of sulfur, as is understandable from the comparison of their spectra in Figure 1.
Unlike aurorae, atmospheric optics or clouds cannot shine by themselves. The light source for clouds is sunshine or solar reflected light, including moonlight (e.g., Capron, 1883;Love, 2018;Stephenson et al., 2019). Therefore, as opposed to auroral emissions, the spectra of such atmospheric optics must inevitably involve dark absorption lines, typical with the sunlight (see Figure 1; e.g., Capron, 1879). As Liais saw this phenomenon with a spectroscope and associated it with aurora, we cannot associate this phenomenon with atmospheric optics, originated from the sunshine or solar reflected lights.

Modern interpretation of the report
There are only a few magnetic field observations that were regularly recorded around the world during the 19th century. The only magnetic indices that can be used for that period are the ak index (Nevanlinna, 2004) and the aa index (Mayaud, 1972 (Jackson et al., 2000). The light purple bar shows the period corresponding to Sanches Dorta's magnetic and aurora observations in Rio de Janeiro during the period 1781-1788 (Vaquero & Trigo, 2005. The green and brown vertical lines mark the Carrington event (e.g., Green & Boardsen, 2006;Hayakawa, Ebihara, Hand, et al., 2018;Hayakawa, Ebihara, Willis, et al., 2019) and the eventual sporadic aurora observation here reported.
is no ak index for that date, but there is aa index for that date. The aa index is a 3hour time resolution magnetic index derived from two magnetic observatories in England and Australia that are nearly antipodal to each other (Mayaud, 1972). The Aa index is then derived from the aa index by taking its daily averages. More detail of these indices can be found in the literature (Mayaud, 1980;Rostoker, 1972). The aa and Aa indices are provided by the British Geological Survey website. Magnetic latitudes are computed by the geomagnetic field GUFM1 model (Jackson, Jonkers, & Walker, 2000) from 1600 to 1990. This model is complimentary to the International Geomagnetic Reference Field (IGRF) model (Thébault et al., 2015). IGRF can compute magnetic fields from 1900 onwards, but GUFM1 can compute magnetic fields as far back as 1590 due to the compilation of a massive data base obtained from observational logs compiled on ships at sea and ports around the world (Jackson et al., 2000;Jonkers, Jackson, & Murray, 2003).
The solid orange line in Figure 2 shows the time evolution of Rio de Janeiro's magnetic latitude (MLAT) from 1600 to 1990. The model shows that MLAT increased from -18.4 • in 1600 to its maximum value (the closest value to the magnetic equator) slightly above -12 • around 1816 when it started to decrease again. The highlighted light purple area (discussed later) corresponds to the 1781-1788 interval between aurorae observed from Rio de Janeiro. The dashed green vertical line marks the Carrington event occurrence (1859), while the dashed brown vertical line indicates the event reported in this letter (1875). with maximum Aa around 27 nT. This magnetic activity is consistent with sunspot number observations recorded a few days before, with very low values and one day with the observation number of 60 (Clette & Lefèvre, 2016;Clette, Svalgaard, Vaquero, & Cliver, 2014). The low magnetic activity conditions during that sporadic aurora event is consistent with the description suggested by Silverman (2003).
Additionally, the results of this study may also explain the reason why great aurora displays observed from Brazil have not been found/reported in the contemporary records for the Carrington event yet. As seen in Figure 3, Rio de Janeiro's MLAT by 1859 was very low, around -12.2 • . While we surveyed auroral reports in Brazilian newspapers during the Carrington event in the BNDigital database, we found only references to great aurora displays and even telegraph system failures in North America and Europe, with nothing being reported as having been observed from Brazil.
Since the equatorward boundary of the auroral oval is reconstructed ∼ 28.5-30.4 • MLAT (Hayakawa, Ebihara, Hand, et al., 2018) assuming aurora height ∼ 400 km (Ebihara et al., 2017;Roach et al., 1960), the expected elevation of auroral visibility would be at best 3 • above the horizon and hence it was quite difficult to observe auroral displays in the sky of Rio de Janeiro (and other Brazilian locations) during the Carrington event. However, it would be worth surveying potential auroral reports in Argentine, Chile, and Uruguay, countries that are located in regions of higher MLATs in South America. Another significant event, the magnetic storm of 4 February 1872, also triggered great aurora displays at low latitudes (Hayakawa, Ebihara, Willis, et al., 2018;Silverman, 2008). Silverman (2008) reported on possible aurora sightings in latitudes as low as 10o or even 3 • , while he casted a caveat on their reliability. The author reported aurora sightings on the French Reunion Island, in the Indian Ocean (21.12 • S, 55.54 • E). We found mention to these aurora sightings during that storm in Brazilian newspapers while searching the BNDigital database, but none occurring from Rio de Janeiro or anywhere else in Brazil.
Another possibility is to interpret Liais optical observations as equatorial plasma bubbles (EPBs), which are structures with depleted plasma density usually formed after sunset in the bottomside ionosphere and move from west to east (Kelley, 2009;Liu, Pedatella, & Hocke, 2017;Mendillo & Tyler, 1983). Since plasma bubbles are faint structures that can be hardly seen by the naked eye (e.g., Wiens, Ledvina, Kintner, Afewerki, & Mulugheta, 2006), Liais event most likely cannot be classified as an EPB event.
It should be mentioned that this is not the first report of an aurora observation performed from Rio de Janeiro, despite its proximity to the magnetic equator. In fact, Vaquero and Trigo (2005) and Vaquero and Trigo (2006) and Carrasco, Trigo, M., and Vaquero (2017) presented a series of magnetic observations and aurora sightings conducted by the Portuguese astronomer Sanches Dorta in the 18th century. According to the authors, the observations conducted by Sanches Dorta, during the period 1781 to 1788, must very likely have occurred during times of elevated magnetic activity. However, according to Figure 2, the MLATs of Rio de Janeiro during these events were around -12.5 • (highlighted purple area). If the events reported by Vaquero and Trigo (2005), Vaquero and Trigo (2006) and Carrasco et al. (2017) were in fact caused by great magnetic storms, their visibility would have reached MLATs closer to the magnetic equator in comparison to the low-latitude Carrington aurorae previously reported (Green & Boardsen, 2006;Hayakawa, Ebihara, Cliver, et al., 2019;Hayakawa, Ebihara, Hand, et al., 2018;Hayakawa, Ebihara, Willis, et al., 2019).
In this letter, we presented for the first time a report on a possible sporadic aurora observation performed from Rio de Janeiro, Brazil, on 15 February 1875. This is the first sporadic aurora report in South America, and the second one in the southern hemisphere (the first observation was reported by Eddie (1894). Additionally, this is the second sporadic aurora observed near the magnetic equator. The original report was authored by Emmanuel Liais, then director of the Imperial Observatory of Rio de Janeiro, and published in the Jornal do Commercio (1875) of the same city. Given the scientific expertise, the contents of scientific descriptions and the experience of the observer, particularly with respect to the use of a spectroscope, Liais' report may be considered credible and possible misinterpretation of the observed phenomenon, such as caused by atmospheric optics (Hayakawa, Vaquero, & Ebihara, 2018;Usoskin et al., 2017), may be discarded. The aurora description presented by Liais is consistent with sporadic aurorae (Abbe, 1895;Botley, 1963;Silverman, 2003). In addition, the very low magnetic latitude of Rio de Janeiro and the weak/mild magnetic activity during the observations are consistent with a previous sporadic aurora observation near the magnetic equator (Hayakawa, Vaquero, & Ebihara, 2018).
Furthermore, in addition to the sporadic aurora causes presented in the introductory section, we speculate that sporadic aurorae may also be caused by the flow of solar wind phase fronts with some inclination in the equatorial plane toward the dusk flank. As suggested by Cameron, Jackel, and Oliveira (2019), such flows of solar wind phase fronts during times of low magnetic activity or quiet conditions would increase magnetic activity over time due to shear and viscosity effects, and the sudden release of this energy may cause sporadic aurorae. More observations and possibly numerical simulations are needed in order to test these hypotheses and advance the knowledge of sporadic aurora triggering.

A The report A.1 Original transcript written in old Portuguese
Rio de Janeiro, 17 de fevereiro de 1875
Observei mais um halo fraco em volta da lua, dentro de um tenue vo de vapor, e como j o assignalou por Bravais, este halo era um tanto mais forte nas intercepes com os raios da aurora. Pouco depois estes raios comearam a encurtar e a retirar-se para o Sul.

A.2 Translation of the original transcript into modern English
Rio de Janeiro, February 17, 1875 Commerce Newspaper Aurora australis Mr. Emmanuel Liais, director of the astronomical observatory of Rio de Janeiro, sent us yesterday the following observations that he made on the aurora australis, on which we already reported: At 7:45pm my attention was caught by a kind of bridal veil spread all over the sky forming a series of white stripes, that started in the south on a circular arc whose center was below the horizon, in the direction of the magnetic needle inclination. The stripes or rays were of such an extension that they passed through the sky from south to north, where they converged at the diametrically opposite point.
Such disposition, producing the form of aurora borealis, made me suppose at once that it could be an aurora australis the phenomenon I witnessed; unfortunately I could not affirm that because of the Moon's presence: moments later, however, I was entirely convinced that was the case, thanks to other circumstances that accompanied it.
In fact, after five minutes of observation, they passed from west to east, and twice successive appearances with a variety of ray intensities occurred, as often happens with aurora borealis and australis. In addition, after a few more minutes the rays, whose intensity had augmented, took in their inferior part a reddish color and in their superior part a faint greenish color that could not result from the effect of the light reflected by the Moon.
I observed then with a spectroscope, where bright lights appeared, the certain evidence of proper lights. All lights belonged to sulfur, a substance that, as is well known, is found in large amounts in the atmosphere.
Then I looked towards the north, where I saw two light bolts, and noticed that small clouds formed in a variable form by effects of condensation and dissolution of vapors. Such clouds moved towards the east direction, slightly to the south, passing below the aurora rays; at the same time the rays decreased in intensity and the colors of the inferior part had disappeared.
I observed one more weak halo around the Moon inside a thin veil of vapor, and as pointed out by Bravais, such halo was somewhat stronger in the interceptions with the aurora rays. Later such rays started to get shorter and move southward.
Then when I left the observation at 8:20pm I sent to the Newspaper the news of this phenomenon. When I went back up to the terrace the numerous rays still existed, however they were shorter and weaker. Around 8:40pm they began to disappear, and at 9:00pm it was possible to see only their vestiges together in the horizon, particularly from east to west.
At 10:00pm two or three rays appeared to be formed once more, but they disappeared later, and small clouds condensed over several points in the sky. Nothing more took place until 3:00am, occasion on which I was called to see two very bright rays that had reappeared towards the east, which caught more attention because of the absence of lunar light.
After diminishing, four rays appeared in the same region, however, weaker in comparison to the first ones, and lasted until the light of the breaking day brought the phenomenon to an end, and at dawn it was shown that the sky was covered by a faint cirrus.
These are the details of the observations, whose deductions will make an object of a special memory.

Declarations
List of abbreviations MLAT: magnetic latitude; LMT: local mean time; GMT: Greenwich mean time; BNDigital: Digital Library of the National Library of Brazil; IGRF: International Geomagnetic Reference Field; EPB: equatorial plasma bubble.

Ethics approval and consent to participate
Not applicable.

Authors contributions
DMO, who is proficient in Portuguese, surveyed the BNDigital database to search for historical accounts of auroral observations from Brazil. He also used GUFM1 to compute magnetic fields and coordinates. HH provided background of space weather events in history particularly with respect to sporadic aurorae. EZ contributed with the interpretation of the historical data and observations in the light of current auroral scientific understanding. AB contributed with interpreting spectroscopic observations. GV provided fundamental information on the ionospheric and magnetic field variations at low latitudes. All authors read and approved the final manuscript.

Consent for publication
Not applicable.

Funding
DMO thanks the financial support of the NASA grants 13-SRITM132-0011 and HSRMAG142-0062, under contract with UMBC. HH acknowledges the JSPS Grand-in-Aid grant JP17J06954, JP15H05816, JP15H05812, and JP15K21709. AB acknowledges the support by the NASA Living With a Star Jack Eddy Postdoctoral Fellowship Program, administered by the Catholic University of America.